Proteins (1. LF UK, NT)

Proteins, or polypeptides, are organic macromolecular substances. Their molecular weight exceeds 10,000. They consist of amino acids in numbers greater than 100. A typical protein contains 200-300.

Construction

peptide binding

amino acids in the peptide are bound to each other by peptide binding. Peptide binding combines with simple covalent binding amino group of one amino acid and a carboxylic group of the other amino acid. The Gibbs energy value of this reaction is equal to G = 10 kJ/mol.

Polycondensation produces an arbitrarily long chain of amino acids. The end of the chain, which has a free (unreacted) amino group, is called the N-end. On the opposite side of the chain, on the other hand, we find a free carboxyl group. This end is called the C-end.

Structure

The structure of proteins is based on the arrangement of amino acids in the chain. Protein structure is very important for their function.

Primary structure

The primary structure is defined by the exact sequence of amino acids in the chain.

Secondary structure

α-helix

β-folded sheet

Secondary structure means the spatial arrangement of amino acids in a chain and stabilization by hydrogen bonds.

They are two basic secondary structures:

1. α-helix: The string is curled into a right-hand helix. The length of a single screw thread is equal to 3,6 amino acid residues. The structure of α-helix is found primarily in fibrous proteins (keratins) or muscle proteins.
2. β-folded sheet: Two parallel and antiparallel chains resembling a folded sheet of paper.

Tertiary structure

The tertiary structure is characterized by other intramolecular binding interactions. For example, disulfide bridges, ionic bonds and van der Waals forces. Other H-bridges may also form in the molecule.

Quarternary structure

The quaternary structure is formed in proteins consisting of two or more polypeptide chains. Their connections provide extramolecular binding interactions with each other. The quaternary structure is found, for example, in hemoglobin. In contrast, myoglobin does not possess quaternary structure.

Protein denaturation

Protein denaturing is the process by which secondary and tertiary structures change. The protein thereby loses its biological activity. Denaturation can be achieved, for example, by heating or changing the pH.

Main nutrients

• peptide bonds
• other bindings
  • disulphide -S-S-
  • ester
  • amide
• Ingredients other than amino acids (physically or chemically)
  • water
  • inorganic ions lipids
  • lipids, sugars, nucleic acid, colored compounds

Clasification

Division By origin animal (meat, milk, eggs) - 60% of food protein vegetable (cereals, legumes, fruits, vegetables) - 30% of food protein non-traditional food protein (seaweed, micro-organisms) By function structural (cell building components, collagen) catalytic (enzymes, hormones) transport (transfer of compounds, myoglobin) motion (muscle proteins, actin, myosin) defence (antibodies, immunoglobulins, lectins) stock (ferritin) sensory (rhodopsin) regulating (histones, hormones) nutritional (source of essential amino acids, source of nitrogen, mass to build and restore tissues) By structure (the presence of a non-protein component) Simple proteins (they contain only a protein chain - globular, fibrillary proteins) globular, spherical proteins (albumins, globulins) fibrillary (fibrous), scleroproteins, stromatic proteins (collagen, keratins, elastins) Compound proteins (contain protein chain and non-protein part - prosthetic group - lipoproteins, glycoproteins) nucleoproteins (nucleic acids) lipoproteins (neutral lipids, phospholipids, sterols) glycoproteins (carbohydrates) phosphoproteins (phosphoric acid) chromoproteins (porphyrin derivatives, flavin) metalloproteins (coordinating boud metals) By solubility Soluble albumins − milk (lactalbumin), egg whites (ovalbumin, conalbumin), wheat (leucosin) globulins − meat (myosin, actin), milk (lactoglobulin), eggs (ovoglobulin) gliadins, or prolamines − wheat (gliadin), barley (hordein), maize (zein) glutelines − wheat (glutenin), rice (oryzenin) protamines − soft roe (cyprimin, salmin, klupein, skombrin) histones − blood (globins of haemoglobin and myoglobin Insoluble collagen, elastin, keratin By status native (natural, biological functions) denatured adjusted (modified, additives) Nutritional aspect full-bodied (essential amino acids in optimal quantities) egg and milk almost full-bodied (some essential amino acids deficient) animal, muscle incomplete (some essential amino acids deficient) all plant, animal connective tissues Food deficient in certain amino acids Lysine − cereals (generally vegetable proteins) Methionine − milk, meat Threonine − wheat, rye Tryptophan − casein, maize, rice Content in food 0–100 % P (in dry matter) animal foods > vegetable legumes, oilseeds > fruits, vegetables eggs – 75 % H2O, 13 % P (whole), 52 % P in dry matter legumes – 12 % H2O, 24 % P (soya 32-45 %), 27 % in dry matter meat (H) – 69 % H2O, 21% P, 68 % in dry matter bread – 38 % H2O, 7 % P, 11 % in dry matter milk – (3,5 % L) 87-90 % H2O, 3,4 % P, 28 % in dry matter potatoes – 78 % H2O, 2 % P, 9 % in dry matter Covering energy needs: ~ 10 % Recommended daily dose: 1-1,2 g/kg Nutrient ratio protein : lipids : carbohydrates (weight = 1 : 1 : 4) energy = < 14 : < 14 : <56 % Physiology and nutrition minimum need for full protein 0,5-0,6 g·kg-1 recommended dose 1,0-1,2 g·kg-1 (not used optimally) ~ 2,4 g·kg-1 growth period, breastfeeding women, recuperation etc. nutritional value (nutritional, biological) total intake Availability of peptide bonds to digestive enzymes Other factors Previously BV (Biological Value) (= g P produced in organism / 100 g P in food) NPU (Net Protein Utilization) PER (Protein Efficiency Ratio) aj. (animals) Depends on: absolute content of essential amino acids relative ratio ratio to non-essential amino acids digestion Today AAS (Amino Acid Score) EAAI (Essential Amino Acid Index) − more accurate AAS (%) = 100 Ai /Asi where: Ai = essential amino acid content in protein Asi = same amino acid content in standard (reference) protein Standard protein = fictitious protein with optimal composition of essential amino acid (AAS = 100%) Physico-chemical qualities solubility, hydration and swelling dissociation optical activity formation of gel formation of emulsions stabilization of foam denaturation physical factors − changes in temperature, pressure, ultrasound, penetrating, penetrating electromagnetic radiation chemical factors− salts, changes in pH (acids, bases), surfactants concequences more accessible to digestive enzymes of digestive tract denaturalization of antinutritional factors, toxic substances (proteas inhibitors, amylas, lectins) inhibition of undesirable enzymes and microorganisms

Meat, meat products, poultry, fish

Meat, meat products, poultry, fish 4 main tissues types (additional blood) epithelial supporting (connective) muscle (transversely striped, smooth) neural Definition Parts of warm-blooded animals in fresh, processed state In the narrower sense: skeletal muscle tissue − number of muscles, bone tendrils, blood supply, nerves, skin, cartilage, bone, fat Other components vitamins free amino acids 0,1–0,3 % taurine (0,02-0,1 %), bile acid component, nerve excitation transfer quaternary ammonium compounds choline 0,02-0,06 %, phospholipids, transmethylation reactions, acetylcholine carnitine 0,05-0,2 %, fatty acid transport guanidine compounds glycogen phosphate sugars and free sugars lactic acid and other acids purines and pyrimidines Use for food and non-food purposes. Myofibrillar proteins muscle fiber myofibrils (contractile fibers) microfilaments (microfibers) myosin actin other proteins In vivo reaction  For more information see Excitation-contraction coupling in skeletal muscle. Post mortem reaction ATP anaerobic glykolysis from glycogen lactic acid › decrease in pH 6,8 to < 5,8 inhibition of glycolytic enzymes Ca2+ / actin reaction with myosin, without ATP › post-mortem stiffening (rigor mortis) Effect on meat quality Maturation of meat cleaving actomyosin by endogenous proteas (mainly cathepsins) cleaving collagen with collagen Meat defects DFD (dry-firm-dark) a DCB (dry-cutting-beef) dark, high binding, low maintenance removal of lactic acid during exsanguination, pH~ 6 PSE (pale-soft-exudative) light, low binding, grey-green surface increased glycolysis stimulated by hormones, pH~ 5,6 Changes in processing ~35° C association of sarcoplasma proteins, reduction in binding, increase in stiffness ~45° C visible changes, shortening=denaturation of myosin ~50-55° C denaturation of actomyosin ~55-65° C denaturation of sarcoplasma proteins, associated structure and gel ~60-65° C changes in collagen conformation (shortening 1/3-1/4) ~80° C oxidation of SH-groups ~90° C gelation of collagen (release of tropocolagen fibres, sol gelatine) ~100 ° C elimination NH3, H2S, other substances, aromatic substances, change of colour Milk and milk products Nutrient content in milk Water by type of milk (origin) 63 - 88 % Complicated dispersion system globular protein of whey − colloidal dispersion casein molecule − micellar dispersion fat − adipose globule (microsomes, φ 0,1-10 µm): emulsion molecules od lipoproteins − colloidal suspension molecular substance (lactose, amino acids, minerals, hydrophilic vitamins) molecular substance (lactose, amino acids, minerals, hydrophilic vitamins) − genuine solution Coloration Protein composition of cow's milk Protein composition of cow's milk Amino acid content of milk caseins α-caseins = phosphoproteins, αS1, αS2, phospherin ß-caseins = phosphoproteins γ-caseins = products of ß-caseins degradation κ-caseins = glycoproteins (2 genetic variants, B), sugar = tetra-, tri-, di-, mono-, GalNAc, Gal, NeuAc, binding to Thr (133) caseins − αS-, ß-, κ-caseins aggreation into submicel and micel, casein moleculs › submicel › micel Changes in storage and processing Heat treatment clumping of fatty globules in raw milk, ~ makroglobulin whey causes proteins to thermable, denature, caseins practically don't denature Pasteurization 72-74 °C (20-40 s): denatures about 50-90 % of serum proteins > 75 °C: most enzymes are inactivated reduction of disulfide-binding elimination of H2S (ß-lactoglobulin) sulphides, disulphides simmer ingestion (Met) degradation of thiamine formation of lactones and methyl ketones sterilisation 140 °C (4 s) denaturation 100 % of the proteins reaction of lactose with lysine loss of lysine (Maillard reaction), fragrances − raw and pasteurised milk ~ 400 fragrances (1-100 mg/kg) Casein clotting and proteolysis fresh milk − pH 6,5-6,75 casein precipitation − pH 4,6 (contaminant, culture microbes) Hard cheeses microorganisims (lactic acid), acidifaction (pH 5,5) proteolytic enzyme rennin (chymosine, rennet), specific hydrolysis of κ-casein − para-κ-kasein = hydrophobic part, part of micells, κ-caseinmacropeptide = hydrophilic part, coagulation curds, (storage › rigidity, acidity, centrifugation of whey, salting, maturation (in the case of Emmental-type cheese, conversion of lactic › propionic acid + CO2), proteolysis, lipolysis › hard cheese Soft cheeses, yoghurts precipitation, low pH (fermentation of lactose, lactic acid), partial coagulation of caseins, in yoghurts is association of micells (gel structure) Insoluble acid casein Sweet casein (rennet clotting) Caseinates (soluble) Na, K, NH4; dispersible: Ca, Mg) Insoluble coprecipitates Whey

Egg

Egg proteins of white 53 %, yolk 47 % nutrient content in hens eggs protein composition of hens egg white and yolk Proteins of egg white ~ 40 proteins (globulins, glycoproteins and phosphoproteins) enzymes (lysozyme, N-acetylmuramide activity, murein, cell walls of bacteria) protein components of enzymes (flavoprotein/riboflavin, avidine/biotine) protease inhibitors (ovomucoid, ovoinhibitor) Concequences viscosity and gel-like consistency of egg white − ovomucoid a ovomucin stability of whipped white foam − ovoglobulins G2 a G3 antimicrobial effects −lysozyme (ovoglobulin G1) antinutritional action − avidine Yolk proteins (emulsion of fat in water) 1/3 = proteins, 2/3 = lipids glyco-, lipo-, glycophospho- and glycophospholipoproteins granules − lipovitellin and phosvitin plasma − lipovitellenin and livetin Changes in storage and processing partial denaturation of white proteins when whipped denaturation by heat 57 °C − beginning 60-65 °C − most proteins denature (not ovomucoid) 65-70 °C − most yolk proteins (not phosvitin)

Foods of vegetable origin

Foods of vegetable origin main sources − plant seeds limited sources − fruits, leaves, tubers and other parts of plants (fruits, vegetable, root crop) Cereals and pseudocereals Basic chemical composition of cereals Cereal proteins and their composition Wheat proteins Flour 7-13 (up to 15) % of proteins 15 % albumins (water soluble) leucosin 7 % globulins (0,4 M-NaCl) edestin 33 % prolamines (70 % ethanol) gliadine 46 % glutelines (remainder) glutenin prolamine/gluteline ratio = 2 : 3 Strong flour = bread (12-14 %) (dough elastic, stiff, necessary intense mixing, retains carbon dioxide, air, bulkier products) Weak flour= biscuit production, confectionery (< 10 %) Dough gluten = viscoelastic mass, 2/3 of water, 1/3 hydrated gluteline (viscosity), gliadines (elasticity), gluten solids = 90 % of proteins, 8 % of lipids, 2 % of sugars Gluten-free products allergic disease - celiac disease (~ 0,05 % of children in Europe) changes in the epithelial cells of the intestinal wall, impaired absorption of nutrients prolamine fraction of wheat, rye, barley, sequence: Pro-Ser-Gln-Gln a Gln-Gln-Gln-Pro limits < 100 mg gliadine/kg (dry matter) Rye proteins no gluten bakery properties: pentoses, certain proteins (bottling in an acidic environment) formation of acids by microorganisms (S. cerevisiae, S. minor, L. plantarum, L. brevis) Legume and oilseed proteins high in globulins, germination function  For more information see amino acid content in legumes. Utilization of non-traditional protein sources Textured herbal proteins Protein-rich preparation

Reactions

Reactions elimination, isomerization, additive, oxidation complex reaction influence of food composition, conditions: temperature, pH, O2, other substances Concequences impairment of biological value breakdown of essential amino acids formation of non-metabolizable products reduction of digestion formation of antinutritional and toxic substances formation of aromatic substances mainly Cysteine, Methionine, Ornitine, Proline amines, aldehydes, alcohols, S-compounds Elimination reaction dekarboxylation (elimination of carbon dioxide) aromatic substances biologically active substances (biogenic amines) histamine (His) , cadaverine (Lys) Elimination of ammonia and water formation of 2,5-dioxopiperazines (cyclical dipeptides) formation of alc-2-enoic acids formation of γ-lactams from γ-amino acids, γ-amino acid Glu, creatine Elimination of function groups of side chains reaction in an acidic environment or thermal reactions protein deamideation, hydrolysis reaction in neutral environment or thermal reactions formation of unusual bindings reaction in alkaline environment or thermal reactions formation of unusual binding, unusual amino acids, D-amino acids (abiogenic) Concequences reduction of digestion reduction in the nutritional value formation of potentially toxic amino acids formation of aromatic substances Acidic environment Production of protein hydrolysates enzymes ofautolysis, yeast autolysates, food hydrolysates, soy sauce acids of food hydrolysates Neutral environment formation of transverse bindings and unusual amino acids ε-amino group Lys, the carboxamide group Asn, Gln Alkaline environment loss of Lysin, Cystein, Serin, Threonin, Arginin aj. 1,2-elimnation of H-X (Ser, Thr, Cys, SySSCy) and hydrolysis X = OH, SH, SR, SSR etc. Cys, Ser›2-aminoacrylic acid (dehydroalanine), Thr › 2-aminocrotonic acid (dehydrobutyrine) addition of functional groups of amino acids (intra- and intermolecular transverse bindings) hydrolysis of cross-linked protein and formation of unusual amino acids, lysinoalanine, lanthionine isomerisation and formation of D-amino acids, reduced reusability Additive reactions carbohydrate reaction (aldehydes, ketones), Maillard reaction coloured substances, aromatic substances, biologically active substances Oxidative reactions oxidative deamination and transamination Enzyme reactions deaminases or transminases, hydrolasses aldehydes − aroma of fruits and vegetables alcohols − aroma of alcoholic beverages (alcohols congeners) Strecker degradation (oxidative decarboxylation) formation of strecker aldehydes non-enzyme reaction Oxidizing agents dicarbonyl compounds carbohydrates quinones inorganic substances (hypochlorites) Formation of other products N- and S-heterocyclical compounds More oxidations oxidised lipids and phenols, O2 (photosensitisers) cysteine and cystine oxidation of Cys na sulfenic, sulfinic, sulfonic (cysteic) acid (unusable) oxidation of Cys to CySSCy oxidation of CySSCy oxidation of Met Reaction with food ingredients reaction with polyphenols dark colour of scrap isolates unusable products, reduced digestion reaction with oxidized lipids unusable products, reduced digestion

References

Related articles

• Peptides (1. LF UK, NT)
• Amino Acids (1. LF UK, NT)
• Amino acids, peptides, protein

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